LONDON—U.S., European and Chinese regulators are likely to give the thumbs up to an alliance of three container-shipping companies, but not before demanding concessions to limit their dominance of the world’s busiest shipping routes, according to people with direct knowledge of the matter.

Denmark’s Maersk Line, France’s CMA CGM and Switzerland’s Mediterranean Shipping Co.—the world’s three biggest container-ship operators in terms of capacity—had planned to start operations of their so-called P3 Alliance in the second quarter of this year, but regulatory approval won’t come until later in the year, these people said. Maersk is a unit of A.P. Moller-Maersk

Senior representatives of the three regulators—the U.S. Federal Maritime Commission, the European Union’s Competition Commission and China’s transport ministry—met in Washington Tuesday to scrutinize the planned alliance.

“The regulators have determined that the P3 isn’t a merger, but an alliance. This means that it will probably be approved, but it will include clauses to protect parties like cargo owners, fuel providers and smaller competitors from price fixing and unfair competition,” said one of the people with direct knowledge of the matter.

Customers of the ship operators—such as freight forwarders, importers and exporters—are campaigning to block the alliance, saying they will have no control in negotiating freight rates with the container-shipping companies. Fuel suppliers are concerned that once P3 is up and running they won’t be able to separately negotiate fuel prices with the three vessel operators. Some smaller container-shipping companies fear they will be unfairly pushed out of the market given that Maersk, CMA CGM and MSC own the world’s largest fleets and biggest individual ships.

“Some of these concerns will be addressed by adding clauses as part of the regulatory approvals. The regulators believe they will reach an agreement with the three ship operators and the P3 will then satisfy all legal prerequisites,” the person familiar with the matter said.

For the three shipping companies, the pact’s logic rests on sharing ships and port facilities from Shanghai to Rotterdam, New York and the U.S. West Coast, as slack global economic growth, stubbornly low freight rates and high fuel costs erode their profit margins.

If approved, the P3 would control an estimated 43% of Asia-to-Europe container shipping, 41% of the trans-Atlantic market and about 24% of the trans-Pacific market.

Chinese regulators, who can take as many as five months to rule on such issues, have yet to receive documents from the three operators necessary for their review, while the Federal Maritime Commission will again look into the matter early this year after sending additional questions to the shipping companies this month, said the people familiar with the situation.